1. Field of the Invention
The present invention relates to a thin film transistor with an LDD region, and, more particularly, to a thin film transistor with an LDD region in which the electrical characteristics, such as the current characteristics, are superior.
2. Description of the Related Art
During fabrication of a thin film transistor (TFT) using polycrystalline silicon, bonding defects, such as dangling bonds, existing on crystal grain boundaries of the polycrystalline silicon included in an active channel region are known to act as a trap for electric charge carriers.
Therefore, the size, size uniformity, number, position, and direction of crystal grains not only have a fatal effect upon TFT characteristics such as threshold voltage (Vth), subthreshold slope, charge carrier mobility, leakage current, and device stability, directly and indirectly, but they also have a fatal effect upon the uniformity of the TFTs, depending on the position of the crystal grains during fabrication of an active matrix display substrate using TFTs.
The number of fatal crystal grain boundaries (hereinafter referred to as “primary” crystal grain boundaries) included in active channel regions of TFTs on the whole substrate of a display device can be equivalent or changed according to the size of the crystal grains, inclination angle Θ, dimension of active channels (length (L), width (W)), and position of each TFT on the substrate (FIG. 1A and FIG. 1B).
As illustrated in FIG. 1A and FIG. 1B, if the maximum number of crystal grain boundaries is Nmax in the number of “primary” crystal grain boundaries included in the active channel regions for the size of crystal grains Gs, active channel dimension L×W, and inclination angle Θ, the number of “primary” crystal grain boundaries included in the active channel regions, according to the position of the TFT on a substrate or display device will be Nmax (3 in case of FIG. 1B) or Nmax−1 (2 in case of FIG. 1A), and uniformity of the most excellent TFT characteristics can be secured when Nmax “primary” crystal grain boundaries are included in active channel regions for all TFTs. That is, the more equal the number of crystal grain boundaries each TFT has, the more excellent uniformity a device obtains.
On the other hand, if the number of TFTs including Nmax “primary” crystal grain boundaries is equivalent to the number of TFTs including Nmax−1 “primary” crystal grain boundaries, it can be easily expected that uniformity is the worst, resulting in undesirable characteristics of TFTs on a TFT substrate or a display device.
As illustrated in FIG. 2A and FIG. 2B, polycrystalline or single crystalline particles are capable of forming large silicon grains on a substrate using a sequential lateral solidification (SLS) crystallization method, and it is reported that characteristics similar to the characteristics of TFTs fabricated of single crystalline silicon are obtained when fabricating a TFT using the large silicon grains.
However, numerous TFTs for driver and pixel arrays must be fabricated to fabricate an active matrix display.
For example, approximately one million pixels are made in fabricating an active matrix display having SVGA resolution, and one TFT is required in each pixel in the case of a liquid crystal display (LCD), and at least two or more TFTs are required in a display using an organic luminescent substance (for example, organic electroluminescent device).
Therefore, it is impossible to fabricate TFTs by growing a certain number of crystal grains, in a certain direction only, for the active channel regions of each of one million to two million, or more, TFTs.
As a method to realize this, technology for transforming amorphous silicon on the whole substrate into polycrystalline silicon, or crystallizing the selected region only on the substrate by the SLS crystallization method after depositing amorphous silicon by PECVD, LPCV, or sputtering, is disclosed, referring to FIG. 2A and FIG. 2B, as disclosed in PCT international patent WO 97/45827.
The selected region is also a considerably large region compared to an active channel region having a dimension of several μm×several μm. Furthermore, the size of the laser beam used in the SLS crystallization method is approximately several mm×dozens of mm, and the stepping and shifting of the laser beam or stage are essentially required to crystallize amorphous silicon of the whole region or selected region on a substrate, wherein misalignment between regions on which the laser beam is irradiated exists. Therefore, the number of “primary” crystal grain boundaries included in the numerous active channel regions of TFTs varies, and TFTs on the whole substrate, or in the driver region and pixel cell region, have unpredictable nonuniformity. The nonuniformity has a fatal adverse effect on the embodiment of an active matrix display device.
Furthermore, it is disclosed in U.S. Pat. No. 6,177,301 that a barrier effect of crystal grain boundaries in the direction of the electric charge carrier is minimized, in the case that the direction of the active channels is parallel to the direction of grown crystal grains by the SLS crystallization method when fabricating a TFT for LCD devices, including a driver and pixel array, by forming large silicon grains using the SLS crystallization method as illustrated in FIG. 3A. Therefore, TFT characteristics second to single crystalline silicon can be obtained. On the other hand, crystal grain boundaries in which TFT characteristics act as a trap of the electric charge carrier exist, and TFT characteristics are greatly deteriorated, in the case that the direction of the active channels is perpendicular to the growing direction of the crystal grains, as illustrated in FIG. 3B.
Actually, there are cases in which TFTs in driver circuits are generally inclined to TFTs in pixel cell regions at an angle of 90° when fabricating an active matrix display, wherein the uniformity of the device can be improved by fabricating TFTs in such a way that the direction of the active channel regions is inclined to the crystal grain growing direction in an angle of 30 to 60° to improve uniformity of characteristics between TFTs, as characteristics of each TFT are not greatly deteriorated, as illustrated in FIG. 3C.
However, there is a probability that fatal crystal grain boundaries are included in the active channel regions since this method also uses crystal grains having a limited size formed by the SLS crystallization method. Therefore, this method has problems in that unpredictable nonuniformity exists, causing differences of characteristics between TFTs.